INVESTIGATING AND UNDERSTANDING THE GROUND – WHY BOTHER?
Dr Jacqueline Skipper Geotechnical Consulting Group
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk WHAT IS “UNDERSTANDING THE GROUND?”
Site Wider area Good enough ground Understanding model for successful geotechnical risks in project order to predict construction generalised risks
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk THIS IS DEPENDENT ON:
• Geotechnical category of project - (importance/shed/house/hi-rise/tunnel?) • Scale of project • The sensitivity of the project
• Greenfield site? • Under or over expensive buildings or infrastructure? • Whose house is it near or under? • Is this a one-off opportunity to get into the site?
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk …PLUS THE GEOLOGY
Why do we have to bother “understanding?” • We have loads of historical geological data, maps • We have in-situ testing techniques and surface geophysics • Do we need all that expensive core and lab testing?
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk “ANYWAY, IT’S MOSTLY LONDON CLAY”
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk HOWEVER, SHORTCOMINGS
• Historical ground information can be wrong/misleading/right but ignored..
• Geological maps are just a ‘good approximation’ e.g. A line on a 1:50,000 map = 50m = site-scale uncertainty
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk AND IF YOU LOOK AT SOUTH AND EAST LONDON – IT’S MOSTLY OTHER GEOLOGY
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk HOWEVER – SHORTCOMINGS (2)
• In-situ testing and geophysics are part of the GI suite but should not be a replacement • Increased complexity of geology makes these harder to interpret • Geophysics and in situ testing alone can be misleading and delay project • We don’t know what we are measuring until we look at samples of the ground
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk EVEN WITH INTRUSIVE GROUND INVESTIGATION AND SAMPLING – WITHOUT UNDERSTANDING, WE ARE STILL GROVELLING ABOUT IN THE GROUND
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk WHY THE GROUND IS SO VARIABLE
Primary (environment of deposition) variability Secondary variability – cementation/weathering soon after deposition Later variability – faulting, Quaternary weathering
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk PRIMARY VARIABILITY
Variability which happens during deposition
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk LONDON CLAY
Up to 200m
Up to 150m
Deep marine environment of deposition Even 20m of sea level change will only vary the sediment a little
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk LONDON CLAY
• Primary variability minimal in London (silty, sandy clay, partings of silt and sand) • Secondary variability – ‘claystone’ formation • Probably more affected by faulting than we think • Good logging very important
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk Up to 300m
Up to 300m
Chalk - slightly variable Very deep marine environment of deposition
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk CHALK
• Low(ish) primary variability • High vulnerability to late stage variability – solution, freeze thaw
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk Lambeth Group sediments. Quite variable
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk Wide range of SPT values compared to London Clay (same area)
Geotechnica 2016 | 6th & 7th July 2016 | BrunelLondon University, Clay, same London area | www.geotechnica.co.uk Swamp/marsh
LATERAL VARIATION IN Sand COASTAL PLAIN AREA, MALAYSIA
Clay soils
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk 3-15m 15-30m
Upnor Formation - Shallow marine, sand flats to estuarine environment of deposition with occasional ash falls
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk 3-15m 5-10m
Sea level fall and uplift of Chalk – Upnor Pebble Beds and shallower marine or estuarine sediments Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk More sea level fall and weathering – Reading Formation Mottled Upnor and Lower Mottled Beds
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk North Sea area
Continuing sea level fall and hot climate EROSION
The MID LAMBETH HIATUS – regionally recognisable erosion boundary Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk Sea Level rises again > Woolwich Formation – clays and shells (Woolwich Formation Lower Shelly Beds) deposited. Gets thinner
Geotechnicawestwards 2016 | 6th & across7th July 2016 London| Brunel University, London | www.geotechnica.co.uk Sea Level starts to fall again > Woolwich Formation – laminated clay/silt/sand (Laminated Beds and Sand Channels)
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk Sea level falls again > Reading Formation Upper Mottled Beds – clayey with small and large channels
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk Sea level rises again > Woolwich Formation Upper Shelly Beds - shallow marine, estuarine, river and lake sediments plus cemented
Geotechnicalayers 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk SECONDARY VARIABILITY
• Weathering that happens soon after deposition • Cementation- e.g. with calcium, iron, silica
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk Upnor Formation emerges above sea level, gets weathered
Lower Mottled Beds (Reading Fm)
Upnor Formation Chelsea Limehouse
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk Even more weathering –cementation with calcium, silica
Chelsea Limehouse
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk Cycle 2: Sea level rises, estuaries form
Rare cemented bits
Lower Shelly Beds, Laminated Beds, Woolwich Fm
Chelsea Limehouse
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk Cycle 2: - More weathering
Upper Mottled Beds (Reading Fm) Lower Shelly Beds, Laminated Beds, Woolwich Fm
Chelsea Limehouse
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk Cycle 3: Sea level rises again, a bit
Common, multiple cemented bits
Upper Shelly Beds, Woolwich Fm
Upper Mottled Beds (Reading Fm) Lower Shelly Beds, Laminated Beds, Woolwich Fm
Chelsea Limehouse
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk “BUT WE’VE GOT OLD DATA FROM THE SAME SITE!”
Increased number Loads spread - shallow of geological strata, piles over wide area each with own risks, multiplies ground risk
Now, loads taken on fewer cores and deeper piles into ?Thanet Sand
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk SAME SITE, ACTUALLY FAULTED
Faulting further • Shallow SI increases • Faults unnoticed in number of London Clay geological risks, multiplies ground risk
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk …FAULTING
• The Lambeth Group is the unit that makes faulting visible • Further complicates existing variability
London Clay (not showing Lambeth Group (showing faulting) faulting)
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk FAULTING AND THE LONDON BASIN – THE VIEW UNTIL QUITE RECENTLY
Central London
12km
30m
Layer-cake strata – mostly London Clay!
Possibly the occasional Nice simple syncline fault
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk North of The South London Coast
Wealden Anticline London Syncline South Erosion Chilterns North Downs Downs Chalk White Cliffs of Dover
Actually, probablyOlder Rocks multiple faults within the folds
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk CROSSRAIL – AT LEAST MAJOR 6 FAULTED ZONES
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk LATER STAGE VARIABILITY
Quaternary (ice age) weathering, faulting etc, last 800,000 years • Sediments of (mostly) cold, some warmer phases in London • Large scale sea level fall and rise - 90m • Periglacial features • Erosion
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk COMPLEX THAMES RIVER TERRACES
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk DRIFT FILLED HOLLOWS – “JUST A HOLE FULL OF GRAVEL”
Recently examined large DFHs show us- • Not just ‘scours‘ • Complex 3D features often surrounded/underlain by weak materials • Potentially involving solution, faults, complex infill materials • May follow faulted zones
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk Ice wedges, Essex
Solution/sinkhole features, Kent
Reconstruction of extent of Devensian ice sheet (dashed line) (Gibbard and Clark 2011 )
Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk AND THE ANSWER IS?
In order to understand normal ground variability we still need: • historical data • well planned, interpreted intrusive site investigations • and laboratory testing, for successful construction
In situ testing and geophysics are excellent tools used in conjunction with the above
14/07/2016 Geotechnica 2016 | 6th & 7th July 2016 | Brunel University, London | www.geotechnica.co.uk 43